1. Field of the Invention
The present invention relates to a laser module for use, for example, as a light source in a wavelength division multiplexing (WDM) communication system.
2. Background Art
Japanese Patent Laid-Open No. 2012-124287 discloses a laser module that outputs laser light of a desired wavelength. Laser modules of a wavelength variable type are being widely used as light sources for WDM communication systems. Digital coherent optical communication as a communication system is in the mainstream. A document: L. E. Nelson, et al., “Performance of a 46-Gbps Dual-Polarization QPSK Transceiver With Real-Time Coherent Equalization Over High PMD Fiber”, IEEE JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 3, Feb. 1, 2009, discloses incorporation of two light sources in one optical transceiver because of the need for transmission light and local light in coherent optical communication.
Incorporation of an optical amplifier for transmission light and an optical amplifier for local light in a laser module for the purpose of achieving reductions in size and cost is conceivable. It is preferable that each of the intensities of output lights from the two optical amplifiers be controlled so as to be constant and not to change with respect to time. From the viewpoint of controlling each of the intensities of output lights from the two optical amplifiers so that the intensity of light is constant, it is preferable to separate transmission light and local light in the laser module and to receive each light with a photodiode. More specifically, two photodiodes and two optical output monitoring circuits for monitoring the intensities of light detected with the photodiodes are provided. However, there is a problem that because of restrictions on the optical system it is impossible to provide two photodiodes and two optical output monitoring circuits.